A typical electrohydraulic steering system comprises a servo cylinder which can be acted upon selectively by a flow of hydraulic fluid in order to transmit a steering supporting force to steerable wheels of the vehicle.
One such electrohydraulic steering system comprises typically a servo pump powered by an electric motor at a speed which will vary between idling and full load depending on the requirements. The flow of hydraulic fluid furnished by the servo pump is applied to a servo valve which supplies the portion of the flow needed to generate the servo force assisting steering to the corresponding chamber of the servo cylinder, the excess flow being led back to the reservoir.
So that in a steering movement the required servo force assisting steering is made available without delay, the servo pump needs to permanently furnish a minimum flow on idling. This results in high energy consumption of the system. In addition, the servo valve results by principle in the flow furnished by the servo pump being throttled by the at least two resistances, thus incurring losses which likewise result in an increased energy consumption.
The object of the invention is to provide an electrohydraulic steering system featuring a simple configuration and cost-effective manufacture.
The invention provides an electrohydraulic steering system comprising a servo cylinder which has two chambers, an electric motor, a servo pump connected to the electric motor, the servo pump being directly connected to the servo cylinder and, depending on the driving direction, being able of pressurizing one or the other chamber of the servo cylinder with a volume flow of a hydraulic fluid, and a control unit controlling the electric motor. The steering system in accordance with the invention thus manages without a servo valve, resulting in for one thing a considerable saving in cost, due to a servo valve being a highly sophisticated component, and for another, saving energy since the throttling losses automatically involved in conjunction with a servo valve are eliminated. Instead of controlling the flow supplied to the servo cylinder by a servo valve, the servo pump is now directly controlled so that it furnishes the necessary flow in each case, i.e. when no servo force assisting steering is needed then the pump is idle. This results in a considerable saving in energy. An additional saving in cost materializes in conclusion by the steering system in accordance with the invention having an extremely simple configuration.
Controlling operation of the servo pump is achieved particularly simply by the control unit signaling the electric motor. For this purpose a closed loop control is used which furnishes the parameters: wanted steering angle, actual steering angle, wanted steering moment and actual steering moment. It is on the basis of these parameters that the speed and/or torque of the electric motor driving the servo pump, preferably a constant delivery pump, is controlled.
In accordance with the preferred embodiment of the invention a reservoir is provided which maintains the hydraulic system at a pressure corresponding to roughly half the maximum system pressure. In this way a high system pressure is already instantly available when the servo pump is started without the servo pump first needing to build up such a pressure. A charging valve, for example, may be provided which opens and closes the connection to the reservoir depending on the pressure in the hydraulic system so that it is automatically maintained primed.
Advantageous aspects of the invention read from the sub-claims.